A purpose of biocides is to prevent the deposition of organic material on different constructions and is often mixed in a suspension (paint) which is applied on the constructions. The mode of performance comprises slow solution of the biocide active compound in water, and it is absorbed from this by simple organisms being present on or close to the surface resulting in intoxication.
Copper was among the first metals which were used in fouling preventive agents for marine applications in a large scale. In the ancient Egypt wooden boats were covered by copper plates to prevent fouling, and the British marine took the same method officially into use in 1762. By the change to steel ships in the nineteenth century it was observed that the copper plates experienced corrosion problems, and they were replaced with copper-containing antifouling. Copper powder, copper-brass powder, cupric hydroxide, cuprous oxide, cuprous thiocyanate and arsenite have all been used in antifouling to a varying extent. It is now generally accepted that cuprous oxide provides the best combination of efficiency, economy and ecological acceptability (H. Wayne Richardson (Ed.): “Handbook of Copper Compounds and Applications”, Marcel Dekker, New York, 1997 (432 p)).
In the work discussed herein attention is paid to the advantages of cuprous oxide in the invention disclosed.
Cuprous oxide can be prepared by electrolysis, pyro-metallurgy or hydro-metallurgy (H. Wayne Richardson (Ed.): “Handbook of Copper Compounds and Applications”, Marcel Dekker, New York, 1997 (432 p)).
Method 1: Copper is heated in the air to temperatures above 1030° C., wherein cuprous oxide is thermodynamically stable. A cooling must take place in an inert atmosphere to prevent further oxidation into cupric oxide. The method results in large lumps of copper oxide.
Method 2: Copper is oxidised in an autoclave at 120° C. and 6 atmospheres in the presence of water, air and small amounts of HCl and H2SO4. The method results in a variable size of particles and density, depending on the pressure and temperature in the reactor.
Method 3: Copper oxide is precipitated by mixing a dissolved copper salt (e.g. Cu2(NH3)4CO3 or CuCl) with NaOH in an aqueous solution. The method results in different particle sizes depending on pH and the temperature of the mixing step.
The efficiency of bioactive compounds depends on several factors: The specific copper compound, the dissolution rate of the copper compound and the persistence of the solution. For environmental reasons it is often desirable to achieve a slow dissolution of the copper compound. This may among others be obtained by increasing the circumference of the elemental particles, i.e. by reducing their total external surface. A reduction of the external surface has also another desirable effect: The particles will be colourless and can be used in paints of different colours.
A conventional copper oxide has elemental particles of a diameter in the range >5 μm. In the last years a new commercial product has entered the market having a diameter <10 μm under the name XLT (extra Low Tint). This product may be produced either through the powderisation of copper oxide prepared by high temperature oxidation or by thermal treatment (sintering) of small elemental particles. Both processes involve the use of temperatures in excess of 1000° C., which implies heavy demands as to the process equipment and materials. Further, both processes result in a variation of particle size and large, respectively small, particles have to be powderised, respectively sintered repeatedly after the first cycle to obtain a satisfying yield of the process.
The aim of the present invention was therefore to provide a product and a process which did not suffer from the above disadvantages.